Optimally with electric motors, if you double the voltage, you are supposed to get double the rpm and double the torque. I'm not sure how much power the PCB lets through.

If 1.2 volts lets through .8 volts
and 2.4 volts lets through 1.4 volts
and 3.6 volts lets through 1.8 volts

then your results will differ, however I've never taken a voltmeter to the motor contacts.